1. Field of the Invention
The present invention is related to a condenser microphone that uses an impedance converter in which a bias diode and resistor are incorporated and, more particularly, is characterized by a current amplifier circuit connected immediately after the impedance converter.
2. Related Background of the Invention
Since the output impedance of a microphone unit of a condenser microphone is high, impedance conversion is performed for output by an impedance converter configured mainly by a field effect transistor (hereinafter, referred to as an “FET”). In some cases, the FET constituting an impedance converter incorporates a bias diode and resistor and in other cases not. A circuit part such as a resistor and diode for applying a bias is indispensable for operating an FET. Therefore, that an FET does not incorporate a bias diode and resistor does not mean that a bias diode and resistor are integrally incorporated with an FET but means that a bias diode and resistor are provided in a form of being externally attached to an FET. When it is necessary for a compact microphone such as a tiepin type microphone to incorporate an impedance converter in a microphone unit section, if an FET is a type that does not incorporate a bias part, a bias part needs to be externally attached to the FET and there arises a problem that the microphone unit section becomes bulky. Therefore, in a compact microphone such as a tiepin type microphone, an impedance converter configured by an FET of type that incorporates a bias resistor and diode is used.
FIG. 3 shows a circuit example of a conventional condenser microphone of type in which an FET does not incorporate a bias part. In FIG. 3, the portion on the left side from line A-A is a microphone head section and the microphone head section comprises an electret condenser microphone unit 1, an impedance converter configured mainly by an FET 2 and converting the impedance of the output from the microphone unit 1, and a bias circuit 3 consisting of resistors, a condenser, and diodes that apply a bias to the FET 2. Symbol 5 denotes a ground line connected to a shield line of a microphone cable and symbols 6 and 7 denote balanced output lines and each of the lines also functions as a phantom power supply line.
FIG. 4 is a graph showing the result of measurement of the relationship between the input level (dBV) and the distortion ratio (%) of the output signal in the conventional example shown in FIG. 3. As for the voltage of the phantom power supply to be supplied to the condenser microphone, the three kinds of voltage, that is, 12 V, 24 V, and 48 V, are specified by RC-8162A (power supply system of a microphone) of the Standard of Electronic Industries Association of Japan (EIAJ), therefore, the respective power supply voltages were supplied and measurement was carried out for the respective voltages. Each of curves P12, P24, and P48 in FIG. 4 shows each result of the measurement carried out at the voltages 12V, 24V, and 48V. As the input level increases, the distortion ratio increases. The input level at a distortion ratio of 1% is 6.12 dBV for a power supply voltage of 12 V, 17.1 dBV for a power supply voltage of 24 V, and not measurable for a power supply voltage of 48 V. In the conventional example shown in FIG. 3, the constant of the bias circuit of the FET 2 is set fixedly, therefore, it is impossible to obtain excellent distortion ratio curves for all of the power supply voltages and in the results shown in FIG. 4, a power supply voltage of as high as 48 V cannot be coped with.
On the other hand, also in the conventional condenser microphone equipped with an FET of type that incorporates a bias resistor and diode as an impedance converter, the bias voltage is fixed by the circuit constant within the FET, therefore, it is impossible to change a drain current. Because of this, it is difficult to operate properly across the entire range of power supply voltage from 12 V to 48 V. In view of this, a condenser microphone equipped with an FET of type that incorporates a bias resistor and diode for proper operation even at 48 V, which is the maximum voltage of a phantom power supply, as shown in FIG. 5.
In FIG. 5, symbol Q1 denotes an impedance converter equipped with an FET that incorporates bias resistor and diodes. Symbol Q2 denotes a transistor connected immediately after the impedance converter Q1 and the transistor Q2 constitutes an emitter follower current amplifier circuit. C1 denotes a capacitor that constitutes the bias circuit of the transistor Q2, R1, R2, and R3 denote resistors that constitute the bias circuit of the transistor Q2, and D2 denotes a constant current diode, respectively.
As described above, the EIAJ standard relating to the power supply system of a microphone specifies the three kinds of phantom power supply voltage and their permissible ranges are specified as 12±1 V, 24±4 V, and 48±4 V, respectively. Therefore, the minimum voltage and the maximum voltage that define the permissible range are 11 V and 52 V, respectively and it is desired for a microphone to operate normally in this range of voltage. In order for a microphone to operate in the above-mentioned range of voltage, priority is given generally in designing a microphone so as to operate at a minimum voltage of 11 V. Because of this, a drawback is presented that the maximum output voltage is kept low. On the other hand, if design is made so that the maximum output voltage is obtained at a power supply voltage of 48 V, another drawback is presented that operation is terminated if a voltage of 12 V or 24 V is connected to the phantom power source.
FIG. 6 shows the result of measurement of the relationship between the input level (dBV) and the distortion ratio (%) of the output signal in the conventional example shown in FIG. 5. If design is made so as to operate at a phantom power supply voltage of 48 V, the maximum output voltage when operation is effected at a power supply voltage of 48 V is 15.3 V and the maximum permissible input sound pressure level when sensitivity is set to −40 dBV/Pa is 149.3 dBSPL. When operation is effected at a power supply voltage of 24 V, the maximum output level is 1.8 dBV and the maximum permissible input sound pressure level when sensitivity is set to −40 dBV/Pa is 142.3 dBSPL. No operation was effected at a power supply voltage of 12 V.
The inventors of the present invention have developed a condenser microphone capable of solving the problems of the conventional technique as described above and filed for patent application formerly (refer to Japanese Patent Application No. 2005-177542). Examples shown in FIG. 7 and FIG. 8 show the examples of a condenser microphone in accordance with the same technical idea as that of the invention relating to the above-mentioned patent application. In these examples, a bias of the transistor Q2 constituting the emitter follower current amplifier circuit connected immediately after the impedance converter Q1 including an FET that incorporates a bias resistor and diodes is applied by a forward voltage of a diode D3. C1 denotes the bias capacitor of the transistor Q2 and R1 and R2 denote the bias resistors of the transistor Q2. Other circuit configuration is the same as that shown in FIG. 5. The forward voltage that appears between terminals of the diode D3 remains substantially constant even if the power supply voltage changes, therefore, the bias of the transistor Q2 remains substantially constant when the power supply voltage changes. The circuit example in FIG. 8 differs from the circuit example in FIG. 7 in that the microphone head section including the condenser microphone unit 1 and the impedance converter Q1 and the power module section including the emitter follower transistor Q2 are separated and the microphone head section and the power module section are connected by a dedicated extension cord. In FIG. 8, the extension cord is shown as three lines in parallel to each another. Further, capacitors for blocking a high-frequency current caused by electromagnetic waves from invading the extension cord are incorporated in the microphone head section and the power module section and inductors are further incorporated in the power module section.
FIG. 9 shows the result of measurement of the relationship between the input level (dBV) and the distortion ratio (%) of the output signal in the conventional example shown in FIG. 7. Operation is effected normally at a phantom power supply voltage of 12 V, 24 V, or 48 V. The maximum output voltage (the voltage at a distortion ratio of 1% ) when operation is effected at a power supply voltage 48 V is 15.3 V and the maximum permissible input sound pressure level when sensitivity is set to −40 dBV/Pa is 149.3 dBSPL. When operation is effected at a power supply voltage of 24 V, the maximum output level is 8.3 dBV and the maximum permissible input sound pressure level when sensitivity is set to −40 dBV/Pa is 142.3 dBSPL. When operation is effected at a power supply voltage of 12 V, the maximum output level is −2.0 dBV and the maximum permissible input sound pressure level when sensitivity is set to −40 dBV/Pa is 132.0 dBSPL.
As shown in the example in FIG. 8, however, if the microphone head section and the power module section are connected by a dedicated extension cord, and capacitors and inductors for blocking a high-frequency current that invades the extension cord are incorporated in the power module, a drawback is presented that the bias of the emitter follower transistor Q2 changes and the operation of the transistor Q2 becomes unstable. In particular, when the extension cord is lengthened, there may be the case where the transistor Q2 operates no longer. Therefore, a condenser microphone of type in which the bias of the transistor Q2 by emitter follower connection to be connected immediately after the impedance converter Q1 including an FET is applied by a forward voltage of a diode is suitable to a microphone of type in which the microphone head section and the power module section are directly connected and not extended by a dedicated cord as shown in the example in FIG. 7.
When extension by a dedicated cord is made, it is necessary to devise so that the bias voltage of the emitter follower transistor Q2 changes when the phantom power supply voltage is switched to another in the power module.
Incidentally, investigation of prior art relating to the application of the present invention resulted in finding no prior art closely relating to the application of the present invention. If obliged to refer to any technique, there is a signal processing device (refer to the patent document 1) having a configuration in which in order to avoid the influence of the click at the time of switching of the phantom power supplies, a microcomputer causes a mute circuit comprised of an analog/digital converter to operate to put the output from the analog/digital converter to zero for a predetermined period of time irrespective of the input signal when switched between power source supply to a microphone from the phantom power source and termination of supply.
The invention described in the patent document 1, however, is not one that devises the bias of the emitter follower circuit immediately after the FET that constitutes the impedance converter.
[Patent document 1] Japanese Unexamined Patent Application Publication No. Hei 9-83274